.box {
    position: fixed;
    top: 0;
    right: 0;
    left: 0;
    bottom: 0;
    background-color: rgba(0, 0, 0, 0.3);
}

.box p {
    width: 300px;
    height: 200px;
    background-color: white;
    margin: 200px auto;
    line-height: 200px;
    text-align: center;
}